During PMR (Perpendicular Magnetic Recording) manufacture it becomes necessary to polish all the magnetic materials away after PPT (partial pole trim) on top of the yoke area. The CMP (chemical mechanical polishing) process must be well controlled in such cases since a magnetic shorting path will be present if under-polish occurs while, on the other hand, the main pole will be damaged if excessive over-polish takes place.
Typically, before polish, one is faced with a stack of metal films covered by a full film of alumina. In this context, the goal of properly executed CMP (Chemical Mechanical Polishing) is to stop at a particular layer that is in the middle of whole stack of other layers so it becomes a critical and challenging task to monitor and control the CMP process in order to make a PMR head successfully.
Referring now to FIG. 1 we show there an example of a magnetic write head that includes a main pole 16, a non-magnetic write gap 15, and a stitched write shield 14, which together surround the coil area 17. Located atop 14 are first and second write shields 12 and 13 respectively. All subsequent figures will be ABS (air bearing surface) views i.e. in the direction shown by arrow 18.
For PMR design, one of the major technology problems is the use of CMP to fabricate the pole structure. Since plated magnetic materials are part of the structure, they must be removed completely at he conclusion of CMP. The prior art CMP process is shown as below, but, as will be seen, one encounters several unique difficulties when doing so.
As seen in FIG. 2, the PMR structure before polishing includes metal stack 14/15/16, which was formed inside photoresist mold 22, as well as field area 21 (made pf magnetic material). The CMP final target is to be stopped at a desired metal interface. If under-polish occurs, a magnetic shorting path will be present. On the other hand, the main pole will be damaged if over-polish occurs. Thus to maintain high yields of functional devices, CMP must be well controlled both in terms of uniformity as well as precise termination when some desired metal layer is reached.
As seen in FIG. 3, once the device and field areas have been formed, photoresist mold 22 is removed and replaced with photoresist layer 32 which extends outwards from the device so as to partially cover field area 21 as well. In this way, device pedestal 14/15/16 is all that remains.
It follows from the above that a monitoring scheme must be provided in order to control the CMP process tightly. However, requiring the CMP process to stop at a middle layer is completely contrary to traditional magnetic head CMP practice. FIB (focused ion beam) is commonly utilized in wafer processing to cut devices and to check where CMP stopped. This has several disadvantages: (1) very expensive due to long off-line cycle time and possible permanent damage the device (2) limited sample size (3) a high quality FIB image is obtained only if the stopping layer shows high contrast relative to other materials under removal by the ion beam which, for most applications, is not the case.
In other words, monitoring and controlling a CMP process in order to make PMR head successfully is a critical and a challenging task.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,063,306, Kaufman et al. disclose two slurries having different selectivities for copper and tantalum. In U.S. Pat. No. 6,554,878, Dill, Jr. et al. show different slurries to polish different materials, including alumina. Boggs et al. describe indicator areas on a wafer to check CMP results in U.S. Pat. No. 5,972,787 and in U.S. Pat. No. 6,226,149, Dill, Jr. et al. disclose the use of CMP in forming a pole while in U.S. Pat. No. 6,024,886, Han et al. teach a CMP process using a polish stop layer.